Mechanical seal assemblies generate a significant amount of heat, and a liquid coolant is typically circulated through the seal cavity of the assembly to remove the seal-generated heat. This coolant is typically circulated through an external circuit having a heat exchanger associated therewith to effect removal of the heat from the coolant. When dealing with a conventional single mechanical seal assembly, the coolant is typically provided by continuously bypassing part of the pump fluid through the seal cavity inasmuch as the cooling demands are generally of less severity, although in some instances a wholly separate coolant is required. When dealing with double mechanical seals, however, the seal-generated heat is typically a much more severe problem, and the pressure within the seal cavity is normally significantly different from the pressure of the pumped fluid, whereby the coolant for a double seal is usually different from the pumped fluid. Further, with ever-increasing operational demands and expectations, and specifically higher rotational speeds, the heat generated by such mechanical seals increases significantly, and thus the cooling demands for mechanical seals have and continue to present a formidable problem.
In view of the above, it has been generally recognized that coolant circulating devices are needed to ensure that the coolant can be properly supplied to and through the seal cavity to effect desired cooling of the mechanical seal and thus ensure control over the temperature thereof. At present, mechanical seal manufacturers typically utilize four different types of devices to effect circulation of coolant within a substantially closed loop (which includes the seal cavity) in an attempt to cool the mechanical seal assembly.
One of the most common and still extensively utilized types of fluid circulating devices consists of an external pump, a reservoir, a pressure regulator, a heat exchanger, pertinent piping and associated gauging and valving. The piping defines a closed loop which connects to and includes the seal cavity, and the external pump forcibly pressurizes and drives the coolant in a circulatory manner around the closed loop. This type of coolant circulating device, however, is obviously structurally and operationally complex, requires an unnecessary and undesirable amount of space, and is costly to install and maintain.
In view of the obvious disadvantages associated with the above type of coolant circulating device, manufacturers have obviously sought less costly and simpler devices, and three such devices have been developed in an attempt to facilitate circulation of coolant, particularly in conjunction with a double seal. These latter three devices are known in the industry as a pumping ring, a cut vane and a pumping screw.
As to a pumping ring, it consists of a serrated ring or similar device attached to the pump shaft for rotation therewith. A fluid exit hole is located through the housing so as to project over the pumping ring, which hole exits either radially or tangentially from the seal cavity. With this type device, the centrifugal force generated by the pumping ring is used to throw the coolant outwardly through the exit hole and thus pump the coolant through the closed loop without requiring external pumps or the like. This type arrangement, however, provides proper cooling only under very limited operational conditions inasmuch as the flow rate of the coolant with this type device is small in relationship to the pressure within the seal cavity, and hence this device has limited capability for extracting seal-generated heat. U.S. Pat. No. 4,466,619, as owned by the assignee of this application, illustrates one type of fluid circulating device employing a pumping ring.
To improve upon the operation or performance of the pumping ring, there has also been developed a cut vane for effecting fluid circulation of the coolant. The cut vane also typically uses a serrated ring like the pumping ring to spin or circulate the fluid within the seal cavity. A stationary housing ring typically closely surrounds the pumping ring. This housing ring defines therein an enlarged channel or space which extends around the pumping ring from an inlet hole to an exit hole, with the region extending from the exit hole to the inlet hole (in the flow direction) being generally closed by a structure which is referred to as the "cut vane". This type device functions by using both the centrifugal force generated by the pumping ring and the velocity head developed as a result of the coolant's peripheral speed. Due to the provision of the cut vane between the exit and inlet holes, the circulating fluid within the enlarged channel is given a significant velocity prior to its striking the cut vane adjacent the exit hole, which cut vane then forces the fluid out the exit hole so that it circulates through a closed loop, passes through a heat exchanger, and is thence resupplied through the inlet hole. With this cut vane fluid circulating device, satisfactory cooling of a mechanical seal can generally be achieved only under limited operating conditions since the rate of coolant flow is relatively small in relationship to the coolant pressure developed within the seal cavity. U.S. Pat. No. 4,560,173, owned by the assignee of this application, illustrates a cut vane in conjunction with a mechanical seal.
The other type of fluid circulating device which has been rather extensively used is a pumping screw. With this arrangement, an elongated pumping sleeve is fixedly and concentrically secured within the rotor in surrounding relationship to the rotatable shaft. The pumping sleeve typically has a spiral pumping groove formed in the periphery thereof so as to screw or "auger" coolant axially along the shaft. With this arrangement, however, the spiral pumping groove and its small cross section creates a severe restriction on the quantity of fluid which can be pumped or recirculated, and hence the effectiveness of this arrangement is seriously questionable. Further, the pumping sleeve occupies substantial space both axially and radially, whereby the overall seal construction oftentimes becomes of undesirably great size, whereby the space and excessive cost of this type arrangement is undesirable. Still further, the effectiveness of this device is wholly dependent on the viscous friction between the spinning screw and the stationary housing, and experience indicates that the performance of this device increases only as the viscous drag increases. This type device works most efficiently with thicker or more viscous fluids, or by increasing the drag at the stationary wall, the latter being accomplished by machining an internal screw thread into the stationary bore in such manner as to be counter to the rotating thread. Such arrangement is also obviously complex and costly. Such pumping screws hence are most efficient only when working with high viscosity fluids, and have only minimal effectiveness when working with low viscosity fluids. Since most mechanical seal assemblies involve low viscosity fluids, the pumping screw has little or only minimal effectiveness in a great majority of use applications.
Accordingly, it is an object of this invention to provide an improved fluid circulating device used in conjunction with a mechanical seal, and particularly for use when handling low viscosity fluids, for permitting effective dissipation of seal-generated heat by effecting improved circulation and specifically rate of flow of coolant within a substantially closed circuit.
According to the present invention, the mechanical seal assembly can be provided with a seal cavity which is significantly larger than the rotating seal parts, thereby providing a diametral clearance around the seal parts which is of significantly greater extent than can typically be utilized, which clearance functions as a chamber in which a substantial quantity of coolant can be circulated around the seal parts. A coolant circulation tube (i.e., an exit tube) projects through the housing into the seal cavity and is provided with a window-shaped opening in the sidewall thereof opposing the direction of coolant circulation. The lower end of the tube is preferably closed off by a bottom wall which is inclined outwardly away from the rotating seal parts in the direction of fluid circulation, and this bottom wall has its lower free edge defining not only the lower edge of the window but also disposed closely adjacent the periphery of the rotating seal parts. The substantial quantity of circulating or spinning coolant, and the substantial velocity head thereof, passes through the window into the inner end of the tube so that the velocity head is converted into a significant static pressure head, whereupon the liquid is confined and is effectively pumped or forced radially outwardly of the tube through a closed circuit having a heat exchanger associated therewith. This circuit at its other end connects to an inlet hole (such as another tube if desired) which communicates with the seal cavity at a location which is circumferentially spaced slightly downstream from the exit tube. The exit tube enables a much greater mass of coolant to be spun around the seal parts by virtue of the larger radial space required as compared to an axial pumping screw and causes a greater mass of coolant to be circulated through the coolant system, thereby permitting creation of a significantly greater flow rate of coolant in relationship to the pressure created within the seal cavity.
Other objects and purposes of the invention will be apparent to persons familiar with structures of this general type upon reading the following specification and inspecting the accompanying drawings.